// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using CommunityToolkit.HighPerformance.Helpers.Internals;
#if NETSTANDARD2_1_OR_GREATER
using RuntimeHelpers = System.Runtime.CompilerServices.RuntimeHelpers;
#else
using RuntimeHelpers = CommunityToolkit.HighPerformance.Helpers.Internals.RuntimeHelpers;
#endif

namespace CommunityToolkit.HighPerformance.Helpers;

/// <summary>
/// Combines the hash code of sequences of <typeparamref name="T"/> values into a single hash code.
/// </summary>
/// <typeparam name="T">The type of values to hash.</typeparam>
/// <remarks>
/// The hash codes returned by the <see cref="Combine"/> method are only guaranteed to be repeatable for
/// the current execution session, just like with the available <see cref="HashCode"/> APIs.In other words,
/// hashing the same <see cref="ReadOnlySpan{T}"/> collection multiple times in the same process will always
/// result in the same hash code, while the same collection being hashed again from another process
/// (or another instance of the same process) is not guaranteed to result in the same final value.
/// For more info, see <see href="https://learn.microsoft.com/en-us/dotnet/api/system.object.gethashcode#remarks"/>.
/// </remarks>
public struct HashCode<T>
    where T : notnull
{
    /// <summary>
    /// Gets a content hash from the input <see cref="ReadOnlySpan{T}"/> instance using the xxHash32 algorithm.
    /// </summary>
    /// <param name="span">The input <see cref="ReadOnlySpan{T}"/> instance</param>
    /// <returns>The xxHash32 value for the input <see cref="ReadOnlySpan{T}"/> instance</returns>
    /// <remarks>The xxHash32 is only guaranteed to be deterministic within the scope of a single app execution</remarks>
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static int Combine(ReadOnlySpan<T> span)
    {
        int hash = CombineValues(span);

        return HashCode.Combine(hash);
    }

    /// <summary>
    /// Gets a content hash from the input <see cref="ReadOnlySpan{T}"/> instance.
    /// </summary>
    /// <param name="span">The input <see cref="ReadOnlySpan{T}"/> instance</param>
    /// <returns>The hash code for the input <see cref="ReadOnlySpan{T}"/> instance</returns>
    /// <remarks>The returned hash code is not processed through <see cref="HashCode"/> APIs.</remarks>
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    internal static unsafe int CombineValues(ReadOnlySpan<T> span)
    {
        ref T r0 = ref MemoryMarshal.GetReference(span);

        // If typeof(T) is not unmanaged, iterate over all the items one by one.
        // This check is always known in advance either by the JITter or by the AOT
        // compiler, so this branch will never actually be executed by the code.
        if (RuntimeHelpers.IsReferenceOrContainsReferences<T>())
        {
            return SpanHelper.GetDjb2HashCode(ref r0, (nint)(uint)span.Length);
        }

        // Get the info for the target memory area to process.
        // The line below is computing the total byte size for the span,
        // and we cast both input factors to uint first to avoid sign extensions
        // (they're both guaranteed to always be positive values), and to let the
        // JIT avoid the 64 bit computation entirely when running in a 32 bit
        // process. In that case it will just compute the byte size as a 32 bit
        // multiplication with overflow, which is guaranteed never to happen anyway.
        ref byte rb = ref Unsafe.As<T, byte>(ref r0);
        nint length = (nint)((uint)span.Length * (uint)sizeof(T));

        return SpanHelper.GetDjb2LikeByteHash(ref rb, length);
    }
}
